1. Field of the Invention
The present invention relates to a four-wheel drive vehicle including front wheels to which a driving force from an engine is transmitted directly, and rear wheels to which a portion of the driving force from the engine is transmitted indirectly through a multi-plate clutch which is brought into its engaged state by a hydraulic pressure generated by a difference between rotational speeds of the front and rear wheels.
2. Description of the Related Art
There is a conventional power transmitting system already proposed by the present assignees in Japanese Patent No. 2516095, wherein, whenever the front wheels are slipped to produce a difference between the rotational speeds of the front and rear wheels, a driving force is transmitted from the front wheels to the rear wheels to automatically switch over the mode of the vehicle from a two-wheel drive mode to a four-wheel drive mode.
The conventional system is designed such that when a difference in rotational speed is produced between the front and rear wheels, a hydraulic pressure is generated by a difference between the amount of oil discharged from a first hydraulic pump operated in operative association with the rotation of the front wheel and the amount of oil discharged from a second hydraulic pump operated in operative association with the rotation of the rear wheels, and such hydraulic pressure causes the multi-plate clutch to be brought into its engaged state, thereby transmitting the driving force from the front wheels to the rear wheels to switch over the mode of the vehicle to the four-wheel drive mode. However, the conventional system suffers from a disadvantage in that there is a time lag until the multi-plate clutch is brought into the engaged state by the hydraulic pressure generated based on the difference between the rotational speeds of the front and rear wheels. Another disadvantage is that, whenever the forward movement of the vehicle is initiated, the front wheels are slipped, resulting in a low responsiveness for bringing the vehicle into the four-wheel drive mode.
The present invention has been derived with the above circumstance in view, and it is an object of the present invention to ensure that in a four-wheel drive vehicle designed so that a multi-plate clutch is brought into its engaged state by a hydraulic pressure based on a difference between rotational speeds of front and rear wheels, the responsiveness from the generation of the difference between the rotational speeds of the front and rear wheels to the shifting of the vehicle into the four-wheel drive mode is enhanced.
To achieve the above object, there is provided a power transmitting system for a four-wheel drive vehicle including front wheels to which a driving force from an engine is transmitted directly, and rear wheels to which a portion of the driving force from the engine is transmitted indirectly through a multi-plate clutch, the multi-plate clutch being brought into its engaged state by a hydraulic pressure generated by hydraulic pumps in accordance with a difference between rotational speeds of the front wheels and the rear wheels. The power transmitting system comprises a torque cam mechanism including a first cam member operated in operative association with the rotation of the front wheels, and a second cam member operated in operative association with the rotation of the rear wheels, the multi-plate clutch being brought into the engaged state by an axial thrust force generated in accordance with a difference between the rotational speeds of the cam members, the torque cam mechanism being arranged so that when the rotational speed of the front wheels is greater than that of the rear wheels during forward traveling of the vehicle, the thrust force is generated, and when the rotational speed of the rear wheels is greater than that of the front wheels during forward traveling of the vehicle, the thrust force is not generated.
With the above arrangement, when the rotational speed of the front wheels is greater than that of the rear wheels during forward traveling of the vehicle, the first cam member and the second cam member of the torque cam mechanism are rotated relative to each other to generate the thrust force, thereby immediately bringing the multi-plate clutch into the engaged state. Therefore, as soon as the front wheels are slipped upon starting of the forward movement of the vehicle or during sudden acceleration of the vehicle moved forwards to generate the difference between the rotational speeds, the driving force is transmitted from the front wheels to the rear wheels. Thus, it is possible to enhance the responsiveness for bringing the vehicle into the four-wheel drive mode to enhance the running performance. Thereafter, the multi-plate clutch is brought into the engaged state by the hydraulic pressure generated by the hydraulic pumps with a small time lag and, hence, a sufficient amount of driving force transmitted from the front wheels to the rear wheels can be ensured. On the other hand, when the rotational speed of the rear wheels is greater than that of the front wheels due to a sudden braking during forward traveling of the vehicle, the torque cam mechanism generates no thrust force. Therefore, it is possible to prevent the driving force from being transmitted from the front wheels to the rear wheels to avoid interference with an ABS system or the like.
One of the first cam member and the second cam member may be connected through a frictional clutch to a member rotated in operative association with one of the front wheels and the rear wheels.
With the above arrangement, one of the first cam member and the second cam member is connected through a frictional clutch to a member rotated in operative association with one of the front wheels and the rear wheels. Therefore, the moment that the relative rotations of the front and rear wheels are produced, the first cam member and the second cam member can be rotated relative to each other to generate the thrust force. Before the multi-plate clutch is thereafter brought into the completely engaged state by the hydraulic pressure, the frictional clutch can be slipped to prevent an excessive load from being applied to the torque cam mechanism.
The member rotated in operative association with the front wheels and the first cam member are connected to each other through the frictional clutch, and the second cam member is fixed to the member rotated in operative association with the rear wheels, so that a hydraulic pressure generated by the hydraulic pumps urges the entire torque cam mechanism axially through an end plate to bring the multi-plate clutch into the engaged state, and a thrust bearing is disposed between the end plate and the first cam member at a location radially inward from an urging portion of the second cam member for urging the multi-plate clutch.
With the above arrangement, the thrust bearing is disposed between the end plate and the first cam member. Therefore, the relative rotations of the end plate rotated in operative association with the front wheels and the first cam member rotated in operative association with the rear wheels after the operation of the torque cam mechanism can be absorbed. Moreover, the thrust bearing is disposed at a location radially inward from the urging portion of the second cam member for urging the multi-plate clutch and hence, the position of the thrust bearing can be displaced radially inwards as much as possible to alleviate the load and to enhance the durability.